Abstract
Results from neutrino mass and oscillation experiments now set the mass of the muon neutrino to less than 2 eV/c2. This fact, together with our former measurement of the muon momentum in pion decay at rest, pμ+=(29.79200±0.00011) MeV/c, allows us to directly determine the charged pion mass with 1 ppm precision which constitutes the most precise value of the charged pion mass to date, mπ+=(139.57021±0.00014)MeV/c2. This value is within 1.44 σ of the Particle Data Group's compilation of the charged pion mass value, mπ±=(139.57061±0.00024) MeV/c2. From pμ+ we derive the kinetic energy of the muon, Tμ+=(4.11984±0.00003) MeV and the mass difference, mπ+−mμ+=(33.91184±0.00014) MeV/c2. From our new mπ+ value, assuming CPT invariance (mπ−=mπ+) and our measured mass difference Dπ=mπ−−mπ0=(4.59364±0.00048) MeV/c2 we obtain a new value for the neutral pion mass, mπ0=(134.97657±0.00050) MeV/c2. One also obtains a new quantitative measure of CPT invariance in the pion sector: (mπ+−mπ−)/mπ(av)=(−2.9±2.0)⋅10−6, an improvement by two orders of magnitude.
Highlights
For the measurement of the muon momentum, we used a single focusing semicircular magnetic spectrometer [8,9] with a homogeneous field
Details of the apparatus are described in Refs. [3, 7]
These measurements were originally intended to determine the mass of the muon neutrino mνμ, or its upper limit, respectively
Summary
For the measurement of the muon momentum, we used a single focusing semicircular magnetic spectrometer [8,9] with a homogeneous field. At the Paul Scherer Institute, PSI, in Villigen, Switzerland, we have measured the muon momentum from pion decay at rest [1,2,3,4,5,6,7]: π + → μ+ + νμ. ***) This value includes the Mark IV result, Ref.
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